Heretofore, compounds having a crystal structure called a “garnet structure” have been known. As one of the compounds having the garnet structure, a compound represented by Y3Al2(AlO4)3 has been known. This Y3Al2(AlO4)3 has been widely known by an abbreviation that is yttrium aluminum garnet (YAG). Then, Y3Al2(AlO4)3 has been widely used for a solid-state laser, translucent ceramics, a phosphor and the like.
Here, the phosphor is a compound that radiates fluorescence by being given a stimulation such as an ultraviolet ray. In the phosphor, when extranuclear electrons of specific atoms which compose the phosphor return to a ground state after being excited by the ultraviolet ray and the like, a difference in energy level between an excited state and the ground state is emitted as visible light. The phosphor is obtained, for example, by containing rare earth ions or transition metal ions (Ce3+, Tb3+, Eu3+, Mn2+, Mn4+, Fe3+, Cr3+ or the like) as an emission center in such a compound as Y3Al2(AlO4)3 having the garnet structure.
Hereinafter, in this description, such a phosphor composed by containing the rare earth ions or the transition metal ions (Ce3+, Tb3+, Eu3+, Mn2+, Mn4+, Fe3+, Cr3+ or the like) as the emission center in the compound having the garnet structure is also referred to as the garnet phosphor. Note that, in this description, the garnet phosphor is simply referred to as a phosphor in some cases. Moreover, the phosphor composed by containing the rare earth ions or the transition metal ions (Ce3+, Tb3+, Eu3+, Mn2+, Mn4+, Fe3+, Cr3+ or the like) as the emission center in Y3Al2(AlO4)3 having the garnet structure is also referred to as a Y3Al2(AlO4)3 phosphor. Furthermore, for convenience, a Y3Al2(AlO4)3 phosphor containing at least Ce3+ as an emission center is also referred to as a Y3Al2(AlO4)3:Ce3+ phosphor.
It is known that the Y3Al2(AlO4)3:Ce3+ phosphor is excited when being irradiated with a particle beam or an electromagnetic wave, such as an electron beam, a vacuum ultraviolet ray and blue light, and radiates yellow green visible light. Moreover, it is also known that the Y3Al2(AlO4)3:Ce3+ phosphor has a 1/10 afterglow time as extremely short as 100 ns or less. Therefore, the Y3Al2(AlO4)3:Ce3+ phosphor is widely used for a large number of light emitting devices.
As described above, a light color of the light radiated by the Y3Al2(AlO4)3:Ce3+ phosphor is yellow green. It is known that, in contrast, when a part or all of elements which compose the Y3Al2(AlO4)3:Ce3+ phosphor are replaced by another element different in ionic radius, a phosphor that radiates a different light color from that of the Y3Al2(AlO4)3:Ce3+ phosphor is obtained.
For example, Patent literatures 1 and 2 disclose a garnet phosphor that radiates a light color with a longer wavelength than that of the Y3Al2(AlO4)3:Ce3+ phosphor. Patent literature 1 discloses a (Y,Gd)3Al2(AlO4)3:Ce3+ phosphor obtained by replacing a part of Y that composes the Y3Al2(AlO4)3:Ce3+ phosphor by gadolinium (Gd). The (Y,Gd)3Al2(AlO4)3:Ce3+ phosphor radiates yellow green to orange visible light, and is widely known as a garnet phosphor that radiates a light color with a longer wavelength than that of the Y3Al2(AlO4)3:Ce3+ phosphor.
However, it is known that temperature quenching of the (Y,Gd)3Al2(AlO4)3:Ce3+ phosphor is increased as the wavelength of the light color to be radiated thereby is increased. In particular, light emission efficiency of the (Y,Gd)3Al2(AlO4)3:Ce3+ phosphor with a light emission peak wavelength of 585 nm or more does not reach a practical level thereof. Herein, the temperature quenching is a phenomenon that light emission efficiency decreases as a temperature of the phosphor rises.
Patent literature 2 discloses a Y3Mg2(SiO4)2(AlO4):Ce3+ phosphor obtained by replacing aluminum (Al) that composes the Y3Al2(AlO4)3:Ce3+ phosphor by magnesium (Mg) and further replacing a part of the (AlO4) group by a (SiO4) group. The Y3Mg2(SiO4)2(AlO4):Ce3+ phosphor is known as a garnet phosphor that radiates orange to red visible light. However, it is known that temperature quenching of the Y3Mg2(SiO4)2(AlO4):Ce3+ phosphor is also large, and light emission efficiency of the Y3Mg2(SiO4)2(AlO4):Ce3+ phosphor does not reach a practical level thereof.
Meanwhile, other than the (Y,Gd)3Al2(AlO4)3:Ce3+ phosphor and the Y3Mg2(SiO4)2(AlO4):Ce3+ phosphor, a garnet phosphor that radiates the light color with a longer wavelength than that of the Y3Al2(AlO4)3:Ce3+ phosphor is also known (for example, Patent literature 3, Non patent literature 1 and Non patent literature 2). This garnet phosphor is obtained by replacing yttrium (Y) by lutetium (Lu) and calcium (Ca), replacing aluminum (Al) by magnesium (Mg), and further, replacing the (AlO4) group by a (SiO4) group. The yttrium (Y), the aluminum (Al) and the (AlO4) group are components of the Y3Al2(AlO4)3:Ce3+ phosphor. This phosphor is a phosphor represented by a general formula: Lu2CaMg2(SiO4)3:Ce3+, and is known to radiate orange visible light and have relatively small temperature quenching.